Model predictive control having application to distillation

Abstract

A method of controlling a distillation column having control valves to control both reflux and the vapor rate within the column. In accordance with the present invention, a temperature sensed in a top section of the column is magnified and utilized within the model predictive controller so that control is more aggressive as temperatures increase beyond a threshold temperature. Additionally, in the distillation column, or in fact in any other system in which two or more manipulated variables control two or more common controlled variables, special modeling techniques are utilized to make controller tuning easier to accomplish. In such modeling techniques, each manipulated variable is assumed to be able to have an effect on a controlled variable by a single step response model and other step response models are utilized so that the other manipulated variable(s) that also would have an effect on the same controlled variable are taken into account by the controller as feed forward variables.

Description

FIELD OF THE INVENTION [0001] The present invention relates to a method of controlling a distillation column or other system by model predictive control. More particularly, the present invention relates to such a method in which temperatures in a top section of the distillation column, as utilized by the controller, are transformed to obtain more aggressive manipulation of reflux addition in columns having instability due to multiple steady-state temperature profiles. Additionally, the present invention relates to such a method that also has application systems other than distillation columns in which certain manipulated variables are utilized within the controller as feed forward variables to allow for simplified controller tuning. BACKGROUND OF THE INVENTION [0002] Model predictive control systems known as “MPC” are utilized to control a variety of industrial processes. Generally speaking, model predictive controllers operate on independent and dependent variables. Independent var...

AI Technical Summary

Benefits of technology

[0010] The present invention provides a method of controlling a distillation column having control valves including a reflux flow control valve to manipulate reflux flow rate to a top section of the column and at least one inlet for a feed to be separated that is situated below a reflux inlet for the reflux flow. The feed has a varying temperature that could potentially have an effect on a first temperature sensed within the top section of the distillation column upon an increase in feed temperature and a varying composition that could also potentially have an effect on the first temperature upon an increase in the less volatile components within the feed.

Problems solved by technology

As a result, during an upset condition caused by a change in feed composition, it can be difficult, to control the liquid to vapor ratio within the column and therefore the product purity.

Another more general problem of control is that model predictive controllers can be very difficult to tune when used in connection with certain types of systems that can include distillation columns.

The difficulty arises in multivariable systems in which movement of each of two or more manipulated variables effect the value of two or more common controlled variables.

Generally speaking, adding reflux tends to cool the entire column.

Under such circumstances, tuning of the controller becomes a time consuming and difficult proposition.

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